Generally, a semiconductor wafer, such as a silicon substrate, is subjected to various processes including a film forming process, an etching process, an oxidizing process, a diffusion process and modification process to fabricate a semiconductor integrated circuit on the semiconductor wafer. When the semiconductor wafer is carried from one processing vessel for the preceding process to another processing vessel for the succeeding process, the semiconductor wafer is exposed to a clean atmosphere. Oxygen and moisture contained in the atmosphere react with active silicon atoms exposed on the surface of the wafer and form a natural oxide film of SiO2. The natural oxide film deteriorates electrical properties. Therefore, the natural oxide film is removed by a wet cleaning process using, for example, an HF solution before subjecting the semiconductor wafer to the next process. A film removing method disclosed in Patent document 1 removes a silicon dioxide film of a different quality selectively with HF gas at a room temperature.
The surface of the wafer from which the natural oxide film has been removed by the cleaning process is highly active. Therefore, if the wafer is exposed to the atmosphere, a natural oxide film (SiO2 film) is formed again. To avoid the reformation of a natural oxide film on the wafer, the wet surface of the wafer from which the natural oxide film has been removed is processed by a chemical process to form a chemical oxide film (SiO2 film) on the cleaned surface of the wafer, the wafer coated with the chemical oxide film is carried to the processing vessel of the next process, and the wafer coated with the chemical oxide film is processed. The chemical oxide film has excellent electrical properties as compared with the natural oxide film and is formed uniformly on the surface of the wafer. Therefore, when the next process is a gate oxide film forming process, a thermal oxide film (SiO2 film) is formed directly on the wafer.
A series of steps of the foregoing processing method of processing the surface of a semiconductor wafer will be described with reference to FIG. 10. It is supposed that the processing method is applied to forming a thermal oxide film (SiO2 film), for example, to be used as a gate oxide film on the surface of a semiconductor wafer.
Referring to FIG. 10(A), a surface of a semiconductor wafer W, such as a silicon substrate, is exposed to the atmosphere. Consequently, a natural oxide film 2 (SiO2 film) having an irregular thickness and inferior electrical properties is formed on a surface of the semiconductor wafer W through the interaction of oxygen and steam (moisture) contained in the atmosphere and silicon atoms. As shown in FIG. 10(B), the semiconductor wafer W is subjected to a wet cleaning process using an HF solution to remove the natural oxide film 2 from the surface of the semiconductor wafer W, the surface of the wafer W exposed by removing the natural oxide film 2 is highly active and a natural oxide film is readily formed again on the wafer W.
To prevent the reformation of a natural oxide film on the wafer W, the wafer W is subjected to a chemical process using a solution prepared by mixing, for example, H2O2 and NH4OH after the removal of the natural oxide film 2 to form a protective chemical oxide film (SiO2 film) 4 by lightly oxidizing the surface of the wafer W as shown in FIG. 10(C). The chemical oxide film 4 is superior in electrical properties to the natural oxide film 2 and is uniform and very thin. The thickness L of the chemical oxide film 4 is in the range of about 0.7 to about 0.9 nm.
Then, as shown in FIG. 10(D), the wafer W is carried to, for example, a thermal oxidation system and is subjected to a thermal oxidation process to form a thermal oxide film (SiO2 film) 6 (refer to, for example, Patent documents 2 and 3). The thermal oxide film 6 is processed by a patterning etching process to use the thermal oxide film 6 as a gate oxide film. The thermal oxide film 6 is sandwiched between the silicon surface of the wafer W and the chemical oxide film 4.
Patent document 1: JP 6-181188 A
Patent document 2: JP 3-140453 A
Patent document 3: JP 2002-176052
The thickness of a film in one layer has been progressively decreased to cope with the further advancement of the complexity of integration and dimensional reduction of semiconductor integrated circuits. Under such circumstances, it is desired to control skillfully processes for forming, for example, gate oxide films to form gate oxide films in a desired thickness in the range of, for example, 1.0 to 1.2 nm.
Although the chemical oxide film 4 has the very small thickness L in the range of about 0.7 to about 0.9 nm, the ratio of the thickness L of the chemical oxide film to the thickness of the gate oxide film, namely, the sum of the respective thicknesses of the chemical oxide film 4 and the thermal oxide film 6, increases when the desired thickness of the gate oxide film is in the range of about 1.0 to about 1.2 nm. The control of the thickness of such a gate oxide film is difficult. Such a problem resides not only in forming the gate oxide film, but also in forming various kinds of thin film.
The chemical oxide film may be removed by the film removing method using HF gas as mentioned in Patent document 1. However, a process singly using HF gas needs to be carried out at a room temperature. It takes much time to change the temperature of a processing vessel, namely, a vertical furnace, having a large heat capacity in a wide temperature range, which reduces throughput greatly.